WO2008037574A1 - Method for adjusting a distance between an electrode and a workpiece - Google Patents

Abstract

The invention relates to a method for adjusting the distance (h) between an electrode (5) and a workpiece (3) in electrochemical machining, wherein the electrode (5) is displaced by means of an oscillating drive system, wherein an acceleration signal (21, 51) of the oscillating drive system is received, the acceleration signal (21, 51) is divided into individual sections (31, 33, 35, 37), characteristic magnitudes of the individual sections (31, 33, 35, 37) of the acceleration signal (21, 51) are determined, the characteristic magnitudes of the individual sections (31, 33, 35, 37) are compared to each other, and the distance (h) of the electrode (5) to the workpiece (3) is adjusted to the predetermined values in case of a deviation of the characteristic magnitudes. The invention further relates to a device for carrying out the method, comprising an electrode (5) that is connected to a drive axis, wherein an oscillating displacement can be transmitted to the electrode (5) via the drive axis. A sensor receiving acceleration signals (21, 51) is provided on the drive axis.

Description

 description

title

Method for adjusting a distance between electrode and workpiece

State of the art

The invention relates to a method for adjusting a distance between an electrode and a workpiece in electrochemical metal machining, wherein the electrode is moved by an oscillating drive system.

Electrochemical metalworking (ECM) allows recesses with a given geometry to be inserted into metal workpieces. In order to avoid that the depression expands radially by dissolution of the walls, the working gap between the electrode and the workpiece is chosen to be as small as possible. Usually, oscillating electrodes are used in this case. These each give off a current pulse when the distance between the electrode and the workpiece is the smallest. The gap between the tool and the electrode is referred to as a working gap.

To control the working gap evaluations of the voltage or the current signal are currently used. Furthermore, methods are known which make use of a measurement of the resistance. These methods each allow a relative measurement and determination of the working gap. They are based on the fact that the measured signals change depending on the size of the working gap. For example, the resistance decreases with decreasing working gap.

Disadvantage of the methods in which the working gap is carried out by evaluation of the voltage or current signal or by measurements of the resistance is that the measured signals depend on the state of the electrolyte in the working gap. For example, during processing, ie during a current or voltage pulse applied to the electrodes, the composition of the electrolyte changes. This change is made, for example, by electrolysis of the water or dissolution of metal ions from the workpiece. This results in a superposition of the effects of approximation, for example the decrease of the measured resistance with decreasing gap size, with effects of the processing itself, for example an additional decrease of the resistance by an increase of the conductivity of the electrolyte by the absorption of dissolved metal ions.

Disclosure of the invention

Advantages of the invention

The method according to the invention for adjusting a distance between an electrode and a workpiece in electrochemical metal working, wherein the electrode is moved by an oscillating drive system, comprises the following steps:

(a) recording an acceleration signal of the oscillating drive system,

(b) decomposing the acceleration signal into individual sections,

(c) determining characteristic magnitudes of the individual sections of the acceleration signal,

(d) comparing the characteristic sizes of the individual sections determined in step (c) with one another,

(e) adjusting the distance of the electrode to the workpiece at a deviation of the characteristic size of the predetermined values.

The advantage of adjusting the distance by evaluating the acceleration signal is that the distance determination is independent of the electrical conditions, for example the conductivity or the processing parameters.

The acceleration signal of the oscillating drive system generally has a simple structure, whereby a simple and robust evaluation logic is made possible. It is exploited that in the recess by the oscillatory motion pressures and thus form forces that counteract the movement of the oscillating drive system. Act. These pressures generated by the system itself are proportional to the distance between the workpiece and the electrode. With decreasing distance, distortions of the acceleration behavior due to the effects of the forces of the drive system and the pressures in the workpiece. These distortions can be easily measured. A conventional oscillating drive system comprises a drive axle on which the accelerations can be detected with the aid of an acceleration sensor.

In a preferred embodiment, the portions in which the acceleration signal is decomposed in step (b) are four quarter curves of one oscillation period. Such an oscillation period is usually sinusoidal, trapezoidal or rectangular. Furthermore, it is also possible that an oscillation period comprises a rest phase in which the electrode is not moved. In a regular oscillation movement usually results in an upper and a lower half-wave. By determining the midpoints of the two half waves, the quarter waves can be determined. In a regular oscillation movement, the amounts of the integrals of the four quarter waves are identical. Also, the envelopes are identical, each of which can be mirrored around an axis. Such a regular oscillation curve, in which the integrals of the four quarter waves have the same magnitude, the envelopes are identical and the derivatives have the same magnitude, is when the distance between the electrode and the workpiece is so great that no pressure forces in the depression act on the electrode. As soon as pressure forces act on the electrode, distortions occur in the second half-wave, i. H. the half-wave at which the electrode reaches its smallest distance to the workpiece.

The characteristic quantities determined in step (c) are preferably determined by integration, differentiation and formation of the envelopes of the quarter waves.

In a preferred embodiment, the characteristic quantities determined in step (c) are compared with predetermined values for determining the distance. The predetermined values with which the characteristic variables are compared can be determined, for example, by a calibration. If the characteristic quantities deviate from the given values, the distance between the electrode and the workpiece can be adjusted. If, for example, the deviation from the characteristic values indicates that the distance between the electrode and the workpiece is too great, this distance can be reduced. -A-

be nert. Accordingly, the distance can be increased even if the distance is too small.

Instead of the calibration by which the predetermined values are determined, it is also possible for the controller to store rule sets, for example the integrals of the quarter waves, for certain quantities, and to carry out a regulation based on these rule sets.

For the calibration for determining the predetermined values with which the characteristic variables are compared, the distance of the oscillating electrode to the bottom of a depression of known depth for different immersion depths is set, for example, and the acceleration signal is recorded at the respective predetermined distance.

The invention further relates to a device for carrying out the method, comprising an electrode which is connected to a drive axle, wherein an oscillating movement can be transmitted to the electrode via the drive axle. On the drive axle, a sensor is arranged, which receives acceleration signals. Preferably, the sensor is connected to an evaluation unit in which the acceleration signals can be evaluated. This makes it possible to achieve an automated adjustment of the distance between the electrode and the workpiece.

Brief description of the drawings

Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.

Show it:

FIG. 1.1 Development of the depression at a small working gap, FIG.

Figure 1.2 Development of the recess at a large working gap,

FIG. 2 is a flowchart of the method as a flowchart,

FIG. 3.1 shows an acceleration signal without distortions, FIG. 3.2 shows an acceleration signal with distortions,

Figure 3.3 Path of the axis,

FIG. 4.1 acceleration signal without distortions with standstill of the oscillation movement, FIG.

FIG. 4.2 acceleration signal with distortions during an oscillatory movement with standstill;

Figure 4.3 shows the path of the axis during an oscillatory motion with standstill.

Embodiments of the invention

Figure 1.1 shows the development of a depression in a workpiece at a small working gap.

In order to produce a recess 1 in a workpiece 3, an electrode 5 is lowered into the workpiece 3. In order for the depression 1 to be able to form in the workpiece 3 by lowering the electrode 5, the workpiece 3 is connected anodically and the electrode 5 is connected cathodically. Between the workpiece 3 and the electrode 5, a current flows, so that the workpiece 3 dissolves in the region of the electrode 5 and the recess 1 is formed. In general, the electrode 5 is moved by an oscillating drive system. As a result, a working gap 7 between the electrode 5 and the workpiece 3 increases and decreases. In each case, at the time when the working gap 7 has reached its smallest distance h, at least one current / voltage pulse is emitted by the electrode 5. As a result, the workpiece 3 on the bottom 9 of the recess 1 dissolves. The recess 1 is further advanced in the workpiece 3.

At a time ti, the depression 1 has a first depth Hi. At a second time t ₂ , which is later than the time ti, the depression 1 has a depth H ₂ . An expansion of the recess 1 takes place at a small working gap 7, as shown in Figure 1.1, not.

FIG. 1.2 shows the development of the depression at a large working gap. At the time ti, the recess 1 has the same shape as at the time ti in FIG. 1.1. However, the at least one current / voltage pulse is emitted by the electrode 5 at a larger working gap 7. As a result, well 1 also widens laterally. At time X ₂ , the depression produced in FIG. 1.2 has the same depth H ₂ as depression 1 at time t ₂ in FIG. 1.1. However, at the time t ₂ in the recess shown in Figure 1.2, the side walls 11 of the recess 1 have dissolved, so that the recess 1 has expanded laterally. For this reason, the machining with a large working gap 7, as shown in Figure 1.2, is not as precise as the machining with a small working gap 7, as shown in Figure 1.1.

For precise machining, it is thus advantageous if the working gap 7 is selected to be as small as possible and the distance h between the workpiece 3 and the electrode 5 can be monitored during the production of the depression 1 and optionally readjusted.

FIG. 2 shows a flow chart of the method according to the invention for adjusting the distance between electrode and workpiece as a flow chart.

In a first step Sl, an acceleration signal is recorded. The acceleration signal is determined, for example, on a drive axis of the electrode 5 by means of an acceleration sensor. However, it is also possible to record the acceleration signal directly at the electrode 5. Due to the oscillating movement of the electrode 5 and the acceleration signal has an oscillating course.

In a second step S2, quarter waves are formed from the acceleration signal for one oscillation period. For this purpose, first the upper and the lower half-wave of the acceleration signal are identified. After identifying the upper and lower half-waves, the midpoints of the half-waves are determined. The midpoints of the halfwaves each separate two quarterwaves.

From the quarter waves determined in step S2, characteristic quantities are next determined. For this purpose, the four quarter waves are integrated, for example, in a step S3 and differentiated in a step S4. In a step S5, the envelopes of the quarter waves are further determined. The characteristic quantities determined in steps S3, S4 and S5 are evaluated in a step S6. The review can for example be done by comparing the calculated values for the quarter wave with each other. Furthermore, it is also possible to use predefined values for the evaluation with which the characteristic variables formed in steps S3, S4 and S5 can be compared. The predetermined values can be determined, for example, by a calibration. In the flowchart shown in FIG. 2, the specification of values is represented by step S7.

Depending on the evaluation carried out in step S6, the process parameters can be modified in a concluding step S8. Thus, for example, if the evaluation shows that the working gap 7 is too large for precise machining, an additional advance of the electrode 5 can take place in order to reduce the working gap 7 again. It is also possible, if the working gap 7 is too small, to increase the distance between the workpiece 3 and the electrode 5 again somewhat. If the working gap 7 is too small, there is a risk that the workpiece 3 and the electrode 5 will touch and a short circuit will occur.

For example, the acceleration signals for several known distances between the electrode and the workpiece are recorded for the calibration for determining the values to be predefined for the comparison with the characteristic variables during the machining. This is done, for example, in that the electrode 5 is retracted into a recess 1 of known dimension until the electrode 5 touches the bottom 9 of the recess 1. Subsequently, the electrode 5 is retracted by a predetermined path. After retraction of the electrode 5, the oscillation movement is started and the acceleration signal is picked up. After the acceleration signal has been recorded, the oscillation movement is ended and the electrode 5 is retracted again by a defined path. The distance between the electrode 5 and workpiece 3 is thereby further increased. Subsequently, the oscillation movement is restarted and the acceleration signal is recorded at the increased distance. This procedure is repeated until a sufficient number of measuring points are available. The number of measuring points depends on the processing situation. The recorded waveforms are then evaluated and it can be created a calibration function.

By the method according to the invention it can also be recognized whether a breakthrough has been generated. The breakthrough detection is based on the fact that the pressure conditions in the recess 1 change suddenly when a breakthrough by the Bottom of the workpiece 3 is formed and the liquid contained in the recess 1 can flow. In order to detect the time of breakthrough, the change in the calculated characteristic quantities is permanently controlled in comparison to previously determined characteristic variables, and in the case of breakthrough, the steps required to achieve a defined workpiece geometry can be initiated. If a breakthrough in a workpiece is to be achieved by the electrochemical metalworking, it can be determined by the breakdown detection, whether the workpiece, which is processed, has the correct dimensions. For example, if the breakthrough by the workpiece occurs too early, it will be recognized that the thickness of the workpiece is too low. Likewise, when the breakthrough occurs too late, it is recognized that the workpiece is too thick. If it is indicated that the breakthrough occurred too early or too late, so can the faulty workpiece singled out.

FIGS. 3.1 to 3.3 show an acceleration signal without distortions, an acceleration signal with distortions and the path of the axis in the case of an oscillatory movement without standstill.

In FIGS. 3.1 and 3.2 the acceleration on the ordinate and the time on the abscissa and the time on the ordinate in FIG. 3.3 and the time on the abscissa are shown.

A course, as shown in FIG. 3.1, results for one oscillation period when the distance between workpiece 3 and electrode 5 is large. Due to the large distance, no pressure forces act on the electrode 5, which exert a visible influence on the acceleration. In the case of a sinusoidal oscillation movement, a sinusoidal profile of the acceleration signal 21 is thus also obtained. The acceleration signal 21 for one oscillation period can be divided into a positive half-wave 23 and a negative half-wave 25. Of the half-waves 23, 25 each of the center is determined. Through the center 27 of the positive half-wave 23, the positive half-wave 23 is divided into a first quarter-wave 31 and a second quarter wave 33. Through the center 29 of the negative half-wave 25, this is divided into a third quarter wave 35 and a fourth quarter wave 37. In a uniform oscillatory motion, the amounts of the integrals of the four quarter waves 31, 33, 35, 37 are identical. Also, the amounts of the derivatives of the four quarter waves 31, 33, 35, 37 are identical. In addition, there are no sign changes in the derivatives of the four quarter waves 31, 33, 35, 37. The envelopes of the four quarter waves 31, 33, 35, 37 are also identical. The acceleration signal 21 shown in Figure 3.2 differs from that shown in Figure 3.1 by distortions 39 in the negative half-wave 25. The distortions 39 lead to different integrals, derivatives and envelopes of the third quarter wave 35 and the fourth quarter wave 37. In the derivatives occur due the distortions 39 sign change on. The strength of the distortions depends on the distance between the electrode 5 and the workpiece 3.

Figure 3.3 shows the path that the electrode 5 travels around a starting point. During an oscillating movement, the electrode 5 is initially moved away from the workpiece 3 until reaching an upper reversal point 43 during a period length 41. At the upper reversal point 43, the movement of the electrode 5 is reversed. The electrode 5 moves toward the workpiece 3. At a lower reversal point 45 where the distance between the electrode 5 and the workpiece 3 is the lowest, the movement of the electrode 5 is changed again and the electrode 5 moves away from the workpiece 3 again , At the lower reversal point 45, the distance between the electrode 5 and the workpiece 3 is the lowest. The current or voltage pulses required for electrochemical metalworking are preferably emitted when the lower reversal point 45 is reached. Even with a small distance between the electrode 5 and workpiece 3, the electrode performs a smooth movement. The distortions 39 that occur in the acceleration signal 21 have no measurable influence on the movement of the electrode 5.

In Figures 4.1 and 4.2 acceleration signals are shown in an oscillatory movement of the electrode with downtime.

The acceleration signals 51 with standstill of the electrode 5 are different from the acceleration signals 21 of the oscillating motion without stopping electrode 5 in that between a negative half-wave 25 and the subsequent positive half-wave 23, the value of the acceleration during the stoppage period is zero. This is represented in the diagram according to FIG. 4.1 by the zero line 53. To determine the characteristic quantities of the stoppage period of the movement signal 51 is ignored. The positive half-wave 23 and the negative half-wave 25, which are picked up by the acceleration sensor while the electrode 5 is moving, become four-quarter waves 31, as in the case of the oscillatory motion with no downtime, as shown in FIGS. 3.1 to 3.3 , 33, 35, 37 decomposed. Of the four quarter waves 31, 33, 35, 37 are again the integrals, derivatives and envelopes formed, which are used as characteristic quantities for the evaluation. As with the movement without downtimes, as shown in Figures 3.1 to 3.3, the acceleration signal 51 is not distorted at a large distance between the electrode 5 and workpiece 3 in the oscillatory motion with downtime. Distortions 39 also occur here when the distance between the workpiece 3 and the electrode 5 is small. This is shown in FIG. 4.2.

The period length 41 also includes the standstill phase in the oscillatory motion at standstill, as shown in Figures 4.1 to 4.3, although this is not used to determine the characteristic quantities.

The path signal of the electrode 5 during an oscillatory movement with standstill is shown in FIG. 4.3. Here it can be seen that the standstill takes place at the lower reversal point 45. Advantage of the standstill of the electrode 5 at the lower turning point 45 is that a longer processing time is possible. It is possible to work more effectively.

Instead of the sinusoidal movement of the electrode 5 illustrated in FIGS. 3.1 to 3.3 and 4.1 to 4.3, the method according to the invention can also be used for any other regular oscillatory movement. Thus, the oscillatory movement, for example, rectangular, trapezoidal, in the form of a rounded rectangle or any other, known in the art form. For the method according to the invention, it is only necessary that a periodic movement of the electrode 5 is present.

Claims

claims A method of adjusting the distance (h) between an electrode (5) and a workpiece (3) in electrochemical metalworking, wherein the electrode (5) is moved by an oscillating drive system, comprising the steps of: (a) receiving an acceleration signal (21, 51) of the oscillating drive system, (b) decomposing the acceleration signal (21, 51) into individual sections (31, 33, 35, 37), (c) determining characteristic quantities of the individual sections (31, 33, 35, 37) of the acceleration signal (21, 51), (d) comparison of the characteristic sizes of the individual sections (31, 33, 35, 37) determined in step (c) with one another, (e) adjusting the distance (h) of the electrode (5) to the workpiece (3) with a deviation of the characteristic quantities from the predetermined values. A method according to claim 1, characterized in that the portions to which the acceleration signal is decomposed in step (b) are four quarter waves (31, 33, 35, 37) of an oscillation period. 3. The method according to claim 2, characterized in that the characteristic quantities are determined by integration, differentiation and formation of the envelopes of the quarter waves (31, 33, 35, 37). 4. The method according to any one of claims 1 to 3, characterized in that the characteristic variables are compared with predetermined values for determining the distance (h). 5. The method according to claim 4, characterized in that the predetermined values, with which the characteristic variables are compared, are determined by a calibration. 6. The method according to claim 5, characterized in that for calibrating the distance (h) of the oscillating electrode (5) to the bottom (9) of a recess (1) with known depth for different immersion depths of the electrode (5) in the recess (1 ) is set and the acceleration signal is recorded at the respective predetermined distance. 7. An apparatus for carrying out the method according to one of claims 1 to 6, comprising an electrode (5) which is connected to a drive axle, wherein via the drive axis an oscillating movement to the electrode (5) can be transmitted, characterized in that on the drive axle, a sensor is arranged, the acceleration signals (21, 51) receives. 8. The device according to claim 7, characterized in that the sensor is connected to an evaluation unit in which the acceleration signals (21, 51) can be evaluated.